Metal oxide primary cells are generally constructed from a pressed metal oxide (e.g., manganese dioxide) cathode, an anode material prepared from zinc powder and an alkaline electrolyte. Usually, the zinc powder is dispersed with the electrolyte in a gel such that more surface area of zinc is available for current production. All of these components are combined within an appropriate casing or housing.
In a typical cell as shown in FIG. 1, the positive current collector is a drawn steel container (2), open on one end and about 0.010 inches thick. The cathode consists of two or more annular rings (5) comprised of a compressed mixture of electrolytic manganese dioxide, graphite and alkaline electrolyte which are placed in contact with the positive current collector. A bead (10) is formed into the container near the open end to support the sealing disk. A separator (4) is placed between the cathode rings and the anode gel (3) which is comprised of powdered zinc, a gelling agent, the electrolyte and mercury. A sealing disk (6) contains a negative current collector (1). The sealing disk (6) and sealant (15), which is applied to the sealing disk or to the container (2), are placed into the open end of the container and in contact with the bead. The open end of the container is crimped over the sealing disk thus compressing it between the crimp of the container on to which the coating (14) has been applied and the bead to seal the cell. An insulation washer (7) with a central aperture is placed over the crimped end of the cell such that the end of the negative current collector (1) protrudes through the aperture. A contact spring (8) is affixed to the end of the negative current collector (1). Terminal caps (9) and (13) are placed into contact with the contact spring (8) and the positive current collector (2), respectively, and an insulating tube (12) and steel shell (11) are placed around the cell and crimped on their ends to hold the terminal caps in place.
Zinc powder is the basic component used to manufacture the anode of primary alkaline cells. The zinc anode is employed in an alkaline solution such as an aqueous potassium hydroxide or sodium hydroxide electrolyte. However, exposure of the zinc to hydroxide ions causes zinc corrosion which results in production of hydrogen gas which may build up.
To minimize zinc corrosion, it has been the practice in the battery industry to use amalgamated zinc in forming the anode. Particularly, amalgamation of zinc with mercury has been found to successfully reduce corrosion and gas build up. Most commercially produced metal oxide cells contain 1%-7% mercury (relative to the weight of zinc).
However, due to the environmental hazard presented by mercury contained in disposed cells, reduction of the amount of mercury in alkaline cells has been an industrial goal. After discharge, cell contents sometimes leak out of the cell as a result of internal gas buildup from electrochemical reactions. Cell contents can also be released when the cell structure is opened or ruptured during or after disposal. As a result, mercury in the cell contents is released and can then be absorbed into the environment. Therefore, reduction of the amount of mercury in such cells will reduce the risk of significant environmental contamination if the cell contents are released.
Many approaches to eliminating the need for mercury have been pursued. In U.S. Pat. No. 4,777,100 corrosion is reduced in aqueous electrochemical cells having zinc anodes comprised of single crystal zinc particles by the addition of small amounts of a gas inhibiting surfactant (e.g., an organic phosphate inhibitor). Published Japanese Patent Application 6084767 (filed Oct. 14, 1983, Matsushita Electric Industrial Co., Ltd.) discloses a zinc alkaline cell which employs a zinc/aluminum alloy with aluminum content 0.02 to 0.50 wt % as mean for reducing mercury levels. Another Matsushita article (Ogawa, "Development of Corrosion Resistance Zinc Alloy and Its Application in Alkaline Dry Battery") and three Matsushita abstract references (Chem. Abs. 110: 216342a, 216343b and 216344c) suggest using indium, thallium, gallium, cadmium, lead, silver, bismuth, calcium, strontium, barium, mercury, silicon and tin as other alloying elements.
However, the addition of organic additives adversely effects cell performance by decreasing cell operating voltage. Furthermore, forming zinc alloys containing aluminum or other materials can significantly add to the complexity and expense of producing anode materials. It would, therefore, be desirable to provide means for reducing zinc anode corrosion while significantly decreasing mercury content without significantly diminishing cell performance or increasing the complexity or expense of manufacturing such cells.